In semiconductor diode lasers, Zn diffusion is typically used to provide a selective low resistance path from the p-metal contact to the active layer. As a result, it is desirable to have the Zn diffusion stop just short of the active layer; otherwise, overdiffusion can result in displacing the p-n junction away from the active layer. Typically, the nominal diffusion depth is obtained by making practice diffusion runs on the edges of one of the laser diode wafers. This method is practical only if the p-side layer thicknesses and compositions do not vary from wafer to wafer or across a specific wafer. However, experience shows that these conditions are generally invalid and the resulting Zn diffusion depths are difficult to control.
One possibility for controlling the diffusion depth is to place an additional layer (a Zn-stop diffusion layer) adjacent to or nearby the active layer through which Zn has a slower diffusion rate than in the p-AlGaAs cladding layer. FIG. la provides an illustration of an AlGaAs semiconductor laser diode which includes a Zn-stop diffusion layer. Upon a n.sup.+ -GaAs substrate 10, there is grown a n-AlGaAs cladding layer 12. On the surface of layer 12 is formed a GaAs active layer 14. Upon layer 14 is formed a p-AlGaAs Zn-stop diffusion layer 16. On the surface of layer 16 is formed the p-AlGaAs cladding layer 18. Upon layer 18 is grown the p-GaAs capping layer 20. Ohmic metals 22 and 24 are deposited on layers 10 and 20, respectively. Using standard diffusion techniques, Zn is diffused into this structure from the top, forming region 26, penetrating, at minimum, into the p-AlGaAs cladding layer. Since Zn diffuses relatively quickly through the p-AlGaAs cladding layer and slowly through the Zn-stop diffusion layer, it becomes simpler to control the Zn diffusion process. U.S. Pat. No. 4,426,703 and U.S. Pat. No. 4,731,789 teach that the Zn diffusivity is an increasing function of the material's Al content (the percentage of Al in AlGaAs, by numbers of atoms). Consequently, p-AlGaAs cladding layers having greater than 40% Al content and Zn-stop diffusion layers of approximately Al.sub.0.15 Ga.sub.0.85 As are taught.
As shown in FIG. 1b is another prior art laser diode having an n.sup.+ -GaAs substrate 28, on which is grown a n-AlGaAs cladding layer 30. On the surface of layer 30 is formed a GaAs active layer 32, upon which is formed a p-Al.sub.0.35 Ga.sub.0.65 As blocking layer 34. Upon layer 34 is formed the p-Al.sub.0.15 Ga.sub.0.85 As Zn-stop diffusion layer 36. On the surface of layer 36 is formed the p-Al.sub.0.45 Ga.sub.0.55 As cladding layer 38. Upon layer 38 is grown a p-GaAs capping layer 40. Ohmic metals 42 and 44 are formed on layers 28 and 40, respectively. Again, using standard diffusion techniques, Zn is diffused into this structure from the top, forming region 46, penetrating, at minimum, into the p-Al.sub.0.45 Ga.sub.0.55 As cladding layer. A problem with the above two designs is that the usage of Zn-stop diffusion layers of low Al-content degrades the performance of laser diodes. If this layer is placed adjacent to the active layer (as in FIG. 1a), then the electrons flowing from the n.sup.+ -GaAs substrate are more easily able to surmount the active layer-p-Al.sub.0.15 Ga.sub.0.85 As potential barrier (since the barrier height is smaller for low Al-content layers), resulting in less stimulated emission in the active layer. If this layer is sandwiched between the p-Al.sub.0.35 Ga.sub.0.65 As blocking layer and the p-Al.sub.0.45 Ga.sub.0.55 As cladding layer (FIG. 1b), then, on account of its index of refraction being larger than that of the surrounding layers, a local peak in the light intensity distribution, centered in the Zn-stop diffusion layer, will be formed (see light intensity distribution accompanying FIG. 1b). Formation of this additional peak results in undesirable output beam qualities and lowers the optical confinement factor in the active layer (leading to higher threshold currents).